The invention relates to an X-ray tube having a rotary anode which is accommodated in a vacuum-tight housing so as to be rotatable by means of at least one metallubricated bearing.
An X-ray tube of this type is particularly suitable as a radiation source in medical diagnostic X-ray apparatuses.
U.S. Pat. No. 4,210,371 describes such an X-ray tube in which the bearing is a sliding bearing of which at least the mutually cooperating bearing surfaces consist of a metal or a metal alloy which is not substantially attacked by the lubricant present in the sliding bearing, which lubricant consists of Ga or a Ga-alloy which comprises at least 50% by weight of Ga and furthermore, not counting impurities, consists of In and/or Sn, which lubricant effectively wets the bearing surfaces and has a vapour pressure which is smaller than 10.sup.-4 Pa at 500.degree. C. "Effective wetting" is to be understood to mean that there is a wetting contact in which a direct interaction exists between the metal atoms of the bearing surfaces and the atoms of the Ga or of the Ga-alloys. The bearing surfaces preferably consist of W or Mo or of an alloy of W and Mo because then no alloys with the lubricant are formed. Mo is preferably used because this is easier to machine than W.
Invenstigations have demonstrated that at the operating temperature of the X-ray tube, i.e. above 300.degree. C., a closed polycrystalline layer or a Ga-alloy may be formed on the Mo bearing surfaces. An example of an intermetallic compound which can be formed on the interface is MoGa.sub.5. As a result of this the bearing may gradually become occluded, which adversely influences the life of the X-ray tube.